The most common and most likely answer to this crossword clue is the 6 plus 8 letter word FILING CABINETS
I've seen this crossword clue in The Sunday Telegraph
F
I
L
I
N
G
C
A
B
I
N
E
T
S
Buddy explains!
The straight part
We know that "They store documents" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case they store documents = filing cabinets.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.